Field of the Invention
The invention relates to a method for adjusting the primary side of an X-ray diffractometer, wherein the primary side comprises a collimator, an X-ray optics, an X-ray source, in particular an X-ray tube, wherein the collimator, the X-ray optics and the X-ray source are mounted directly or indirectly on a base structure, and wherein the orientation and position of the X-ray optics and the position of the X-ray source are adjusted relative to the base structure. Such a method may be used with the high-flux X-ray source known from EP 1 462 794 B1.
Description of the Related Art
X-ray diffractometry has been used for several decades for structural determination of matter with atomic resolution. The samples used in single crystal diffractometry are very small, often smaller than 100 μm. A typical single crystal diffractometer consists of the following components: X-ray tube for generating X-ray radiation, X-ray optics for conditioning the X-ray radiation, video microscope for centering the sample, exposure shutter for controlling the sample exposure, collimator for limiting the X-ray beam cross section, goniometer for orientation of the sample, primary beam attenuator for absorption of the direct beam and a planar detector for measuring the X-ray radiation diffracted by the sample.
Single crystal samples are often available only in a very small size; sample diameters of 100 μm or less are often encountered. To be able to perform measurements on such a small sample, a very precise conditioning and adjustment of the X-ray beam used are necessary.
X-ray beams for single crystal diffractometry are typically generated by an X-ray tube, whose source focus is imaged with X-ray optics, for example, a Montel mirror. The beam conditioned by the X-ray optic is directed through a collimator at the sample. The diffracted X-ray radiation is typically detected with a CCD camera. The sample is aligned by means of a goniometer. The X-ray tubes used in single crystal diffractometry are typically point-focus sources having a round optical focus with a diameter of approximately 50 μm. Stationary microfocus tubes, rotating anodes or liquid metal sources are used as the X-ray tubes.
The X-ray optic is usually a multi-layer X-ray mirror, so-called Montel mirror (e.g., U.S. Pat. No. 6,041,099 A). These are mirrors coated with gradient multi-layers. Due to the parabolic or paraboloid and/or elliptical or ellipsoid surface, they collimate and/or focus the X-ray radiation generated in the X-ray tube. In addition, these X-ray optics act as monochromators. In addition to the conditioning, the X-ray optics should also deflect the X-ray radiation onto the sample. Because of the small sample sizes in single crystal diffractometry, focusing X-ray optics are typically used for this purpose. The goal is to strike only the sample, despite the small sample size, because otherwise it leads to interference radiation and can have a negative effect on the quality of the data recorded by the planar detector.
Typical reflex widths of Montel mirrors are approximately 1 mrad (=0.057°) for Cu-Kα X-ray radiation and/or approximately 0.5 mrad (=0.028°) for Mo-Kα X-ray radiation. The angle range in which the mirrors reflect optimally is thus relatively narrow. The accurate adjustment of the source and/or source focus and the X-ray optic is therefore very important for typical focus sizes of 100 μm. Even a minor misalignment results in significantly inferior X-ray beam properties on the sample.
A single-hole aperture collimator has a pinhole aperture directly in front of the sample with the purpose of limiting the dimensions of the X-ray beam so that it illuminates only the sample. Typical pinhole aperture sizes here are 100 μm or 300 μm. A two-hole aperture collimator additionally has a pinhole aperture as close to the mirror output as possible, with the goal of reducing the maximum divergence of the X-ray beam. Typical pinhole aperture sizes are 500 μm at the input end and 300 μm at the output end (as seen in the direction of the beam).
The aforementioned components must then be adjusted relative to one another so that the X-ray optic optimally conditions the X-ray radiation output from the X-ray tube. The monochromatized X-ray beam shaped in two dimensions perpendicular to the beam propagation direction must additionally pass through the exposure shutter and the collimator, so that only the sample is illuminated with a maximum X-ray beam intensity. In addition, the X-ray beam exciting the sample must be aligned, so that it strikes the detector at a right angle.
For alignment of the components to one another, there are typically multiple adjusting screws on the components (e.g., U.S. Pat. No. 7,511,902 B2) according to the prior art. Furthermore, the adjustment process is carried out with X-ray radiation from the beginning. Since the safety requirements of today's X-ray systems have increased, adjustment work is allowed to be performed on the open beam only to a very restricted extent or not at all. Therefore, the diffractometer is typically situated in a radiation-safe radiation protection box, which ensures that the effective beam and/or the stray radiation emitted by the sample is/are blocked by suitable absorbers such as lead glass. Since the entire adjustment process is carried out with the help of X-ray radiation, the safety shutter must be closed for each adjustment strip, i.e., the actuation of an adjusting screw. A typical adjusting step thus involves the following procedure: 1) closing the safety shutter, 2) opening the door of the radiation safety box, 3) turning the adjusting screw, 4) closing the radiation safety box, 5) opening the safety shutter, 6) exposing the detector to the X-ray radiation, 7) reading out the detector and determining the count rate. The entire process typically takes 30 sec for a single adjustment step.
According to the prior art, the adjustment process on the primary side of a single crystal diffractometer includes the following typical steps: 1) adjusting the Montel mirror at the source focus with subsequent maximization of intensity, 2) adjusting the position and/or orientation of the X-ray beam and the collimator relative to one another, so that the X-ray radiation at the output end of the collimator is visible, 3) maximizing the intensity at the output end of the collimator, 4) aligning the X-ray beam with the sample. Now if the X-ray beam does not strike the detector at a right angle, the orientation of the Montel mirror must be changed to adjust the orientation of the X-ray beam accordingly. The adjustment process begins again at step 1). For each of the steps mentioned above, a plurality of the individual adjustment steps described above must be carried out on the respective adjusting screws. Finally, when the source focus is set for the Montel mirror, so that a maximum photon flux strikes the sample downstream from the collimator and strikes the detector at a right angle accordingly, then another sample crystal is measured.
With such an X-ray diffractometer design, adjustment of the primary side, i.e., the components before the sample, is difficult and time consuming and is usually done only by specially trained personnel of the diffractometer manufacturer. A relative adjustment of the components is carried out here essentially by maximizing the beam intensity at the output end in a plurality of individual adjustment steps. Based on safety regulations that are in effect, the individual adjustment steps may be carried out only when the X-ray beam is interrupted and/or the intensity measurements are performed only with the safety housing closed, which increases the adjustment effort to a particular extent. The adjustment is particularly problematical when no X-ray intensity passes through the collimator at the start of the adjustment because of a great misalignment. Since there are so many degrees of freedom, it is unclear which adjustment positions must be altered and in which way in order to achieve intensity again. The adjustment of the primary side of an X-ray diffractometer frequently takes several days.
In the typical adjustment process described above, steps 1) to 3) in particular are very time intensive and tedious. If there is a great misalignment between the source focus and the Montel mirror, the X-ray optics will reflect practically nothing. Since it is not clear without reflection on the X-ray optics where the fitting relative position and/or orientation is/are to be found, a great deal of time is required to find this through successive changes in the adjusting screws.
Since the X-ray optics and thus the X-ray beam conditioned by it have a relatively arbitrary position and orientation, the X-ray beam will in all probability not pass through a collimator mechanically positioned at the center. This means that the position and/or orientation of the X-ray beam and thus those of the X-ray optics must be adapted to the position of the collimator. By successively varying the beam position, thus the plane of the collimator aperture is scanned in a tedious process to detect an X-ray beam at the output end of the collimator. If this scanning takes place, for example, with a mirror housing according to U.S. Pat. No. 7,511,902 B2, then the adjusting screws act on the mirror backs by way of cams. The resulting movement thereof is thus not linear but instead is sinusoidal, which makes the change in position of the output beam from the alignment with the collimator aperture less intuitive and thus time consuming. Furthermore, it may happen that, for example, the Montel mirror is too far out of adjustment, so that the X-ray mirror no longer creates a reflection, which leads to a further loss of time.
Since each individual adjustment takes approximately 30 sec, one easily arrives at an adjustment expenditure according to the prior art amounting to several days. In the worst cases, the adjustment time may be as long as a week. This condition is not encouraging and makes clear the demand for an improved and simplified adjustment concept.
EP 1 462 794 B1 describes a high-flux X-ray source, in which an X-ray optic is mounted on an X-ray tube in a housing wherein the housing can be tilted and shifted with respect to the X-ray tube to adjust the housing by means of four adjusting screws. In addition, a collimator that can be tilted by means of two adjusting screws for the adjustment is mounted on the X-ray tube. The X-ray tube can also be adjusted vertically and horizontally by means of five screws with respect to a pedestal.
DE 11 2013 002 039 T5 describes an X-ray analyzer in which the position of a multi-layer mirror on the primary beam side can be adjusted by means of a motor. In addition, sensors to detect the position of optical X-ray components are proposed.
EP 1 365 231 A2 describes an X-ray diffractometer, wherein an X-ray source and a monochromator are firmly pre-assembled to each other.